The invention relates to lasers, especially passively mode-coupled, continuously operated lasers.
Lasers generally have a so-called optical resonator and an amplification medium. In the most simple case, the optical resonator consists of mirrors designed and oriented such that a light wave can continuously be reflected back and forth between them. Similar to a vibrating string, this is not possible with just any light wave, only with certain specific ones defined by the resonator. They are referred to as resonator modes.
The emission of laser light causes the withdrawal of energy from the light wave in the resonator. In order to compensate such losses the light wave passes through the amplification medium provided in the resonator. Each amplification medium intensifies the light only in a certain wavelength range. Therefore, the laser can only emit light which is both continuously reflected back and forth, thus equivalent to a resonator mode, and which is efficiently amplified at the same time. Consequently, only the light from selected resonator modes is emitted.
When the light wave passes through the amplification medium, it takes energy away from the latter, which has to be supplied again, xe2x80x9cpumpedxe2x80x9d back in from the outside. If the energy is not supplied to the amplification medium as quickly as it is removed when the light wave passes through, a pulse-like laser operation can ensue.
Such a macropulse will not occur until sufficient energy can be withdrawn from the amplification medium.
Frequently, the emission of especially short light pulses is desired. This can be achieved when as many modes as possible occur virtually simultaneously, overlaying each other such that the desired short pulse is obtained.
This can be achieved by means of the so-called mode-coupling, which ensures that when a first of the resonator modes begins to build up, others will follow. In practical application, an element can be provided in the resonator, for example, which absorbs or reflects to varying degrees depending on the strength of the incident light. A known such element is a so-called saturable semiconductor absorber mirror; see U. Keller, K. J. Weingarten, F. X. Kxc3xa4rtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Hxc3x6nninger, N. Matuschek and J. Aus der Au xe2x80x9cSemiconductor Saturable Absorber Mirrors (SESAMs) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers, IEEE Journ. of selected topics in Quantum Electronics, vol. 2, No. 3, September 1996, page 435. The reflectivity of said element increases when the incident light becomes more intense. The result of such an element in the resonator is that whenever the light wave pertaining to a first resonator mode is especially strongly reflected, this will also be the case with other resonator modes. Therefore, the resonator modes are said to be coupled, and accordingly this is referred to as mode-coupling.
A problem in many applications is that a number of interferences or impairments and/or instabilities can occur with common lasers, including the so-called Q-switching (i.e. the development of macropulses), relaxation oscillations and spiking (switching instabilities). In particular, this results in a very high fluctuation in the average laser output. Such interferences can occur under certain operating conditions, for example at a certain pumping power. The fluctuation typically has frequencies ranging between 10 kHz and a few megahertz. The fluctuations can also occur non-periodic, i.e. irregularly, which is especially troublesome. Moreover, the sudden very high intensities in the resonator can destroy or shorten the lifetime of the resonator-internal or other components.
This is especially problematic with passive continuously mode-coupled lasers, lasers with resonator-internal frequency shift and/or resonator-internal frequency doubling and/or lasers where only every nth pulse is coupled out, which then has a nth times higher pulse energy. The latter lasers are called xe2x80x9ccavity-dumpedxe2x80x9d lasers.
Such fluctuations, i.e. the lack of or insufficient potential for adequate stabilization in a technically relevant range, are often the reason why the use of the lasers, i.e. laser systems, is highly limited, why certain applications are not available at all and/or why certain lasers cannot operate in certain operating modes, such as continuous wave operation or mode-coupled continuous wave operation (cw mode locking). Applications worth mentioning, for example, are related to telecommunications, material processing, optical scanning (sampling), etc., which were previously not available, at least in part.
On the other hand, there are applications where it is desired to decouple pulses with an especially high pulse power from the laser.
Other problems occur when the time sequence of the individual pulses has to remain stable, especially in Q-switched laser systems.
The prior art includes various approaches to solve the above problems, but they have to be considered inadequate.
For example, in xe2x80x9cSemiconductor nonlinearities for solid-state laser mode-locking and Q-switchingxe2x80x9d (Nonlinear Optics in Semiconductors II and Semimetals, vol. 59, 211-285, 1999), U. Keller provides an example of how maximum energy can be coupled out of the pulse string of a mode-coupled laser by means of controlled Q-switching.
Other attempts were made by setting various parameters that co-define the laser emission so as to allow a stable continuous mode-coupled laser, i.e. laser system operation, at least in certain ranges of the available pumping power. Among others, the laser materials, amplification media, absorber systems, laser mode cross-sections, etc. were varied. For example, we are referring to E. R. Thoen, E. M. Koontz, M. Joschko, P. Langlois, T. R. Schibli, F. X. Kxc3xa4rtner, E. P. Ippen, L. A. Kolodziejski in xe2x80x9cTwo-photon absorption in semiconductor saturable absorber mirrorsxe2x80x9d, Appl. Phys. Lett. 74, 3927-3929, 1999, relating to special nonlinear absorber elements.
One problem, which remains unsolved by the above approaches, is that stabilization is achieved only within a limited parameter range of the systems. This can have a negative effect when the laser is turned on and/or powered up and in longer operation.
Furthermore, the proposed absorption processes, for example the two-photon absorption, restrict the maximum achievable pulse energy, for example. The absorbers are exposed to very high stresses and the lifetime is limited because of the high saturation which accompanies the deposition of high outputs in the material.
The aim of the invention is to provide an innovation for industrial application.
In accordance with a first substantial aspect of the invention, it is proposed for a laser with a decoupling device for emitting a laser output depending on at least one influenceable parameter and a mode-coupling device for coupling a plurality of the laserable modes of the resonator, to provide a detector for detecting a value related to the laser output, especially the emitted laser output, and a parameter varying device for varying the at least one parameter in response to the detected value.
By means of the claimed method even high-frequency fluctuations can be controlled. Therefore, for the first time it was found that with the above type of laser systems high-speed instabilities, especially Q-switching instability, spiking and relaxation oscillations can be controlled to the net laser gain by means of coupling the laser output. This applies to very broad operating ranges. Said very broad operating ranges began, at least in substantial practical exemplary embodiments of the invention, at or at least close to the laser threshold and extended, especially continuously, up to a significant laser output where both the emitted output and the supplied pumping power represented a multiple of the respective output just above the laser threshold.
The invention allows higher repetition rates than previously. Furthermore, higher pulse energies than before can be set in continuous mode-coupled operation, because the beam cross-section in the amplification medium and/or on the saturable absorber can be set higher without risking undesired Q-switching.
For a practical implementation of the invention, it is proposed to vary a parameter influencing the internally circulating and/or the decoupled laser output. A substantial advantage is that in many existing systems no or no significant modifications are required in the optical structure or the resonator.
It should be noted that the above allows a stabilization of the laser output and, if instead of a parameter variation with negative feedback effect, the parameter is varied so as to have positive feedback, a specific destabilization can be achieved.
It should be noted that the parameter variation can also be used for initiating a mode-coupling process in that the parameter varying device is supplied with a suitable trigger pulse.
The latter is especially advantageous for intensifying a Q-switching instability. Such a specific influence on the Q-switching instability can be used for emitting the high pulses generated by Q-switching at certain times. With the aid of an approximately quartz-stabilized signal generator a stable repetition rate can be obtained. The generator signals and the detected value that is representative for the laser output are preferably added in this case. Considerably higher pulse energies and/or repetition rates than those commonly possible can be achieved.
According to an especially preferred example the, or one of the parameters influencing the decoupled laser output is the pumping power. In this case, the influence on the parameter, especially the control, can be implemented especially easily and by means of a substantially purely electronic modification of common laser systems. Therefore, especially in diode-pumped laser systems said variant is particularly easily implemented, because in this case, only the pump current supplied to the laser pump diodes has to be modified. Alternatively, a loss modulation can take place, which means a loss modulator is provided within the resonator whose loss is varied in response to the detected value.
It is possible and preferable to design the parameter varying device such that it varies the parameter so as to even out the emitted laser output, not only the laser output circulating in the resonator.
According to a preferred variant, the control can be such that the parameter varying device evens out the output averaged over at least one resonator roundtrip time. This is possible even when the control works comparatively slowly, especially compared to the resonator roundtrip time, for example because a slow and therefore quasi integrating photo-element is used for detecting the value related to the laser output. A means for integrating and/or averaging high-speed variations of the value related to the laser output is then provided, where the parameter varying device is then designed to vary the parameter in response to the averaged and/or integrated value.
It is possible and preferable to stabilize the output to less than 5 to 10% of the variations occurring without stabilization. According to an especially preferred exemplary embodiment a stabilization of clearly better than 1% was achieved.
The parameter can be varied by different methods in response to the deviation of the detected value from a target value. Especially linear variations relative to the deviation are possible. Alternatively, PID (proportional, integral and/or differential) behaviors can be realized and/or a nonlinear variation can be achieved.
According to a preferred variant, mode-coupling is passive, for example via an element in the resonator which absorbs and/or reflects depending on the output.
Especially an at least partially saturable absorber can be provided, for example.
It is possible that the resonator is provided with a resonator-internal frequency doubler and/or that a loss modulator is provided whose loss is the, or one of the variable parameters. Such a loss modulator can be modulated especially mechanically, acoustically, optically and/or electrooptically. Providing a loss modulator is especially preferred in the laser systems in which the pumping power cannot be varied particularly easily.
The laser can be a system which exhibits spiking behavior if the at least one parameter is not varied in response to the detected value, and/or a system designed for cavity dumping.
The invention also claims a method, for stabilizing a laser with a mode-coupling means for coupling a plurality of the laserable modes of the resonator and a detector for detecting a value related to the emitted laser output and a parameter varying device for varying the at least one parameter in response to the detected value, including the following steps: a value representative for the emitted laser output is detected and the laser output is controlled by varying the at least one parameter such that the emitted laser output evens out.